1. Field of the Invention
The present invention relates to a solid-stated image pickup apparatus and a signal processing method therefor advantageously applicable to the processing of image signals output from, e.g., an image pickup apparatus using complementary colors.
2. Description of the Background Art
Today, a digital camera using a solid-state image pickup apparatus is extensively used because of various advantages available therewith. For example, a digital camera outputs a picture with resolution comparable with the resolution of a camera using a silver halide photosensitive type of film. Also, a digital camera allows a picture picked up to be immediately seen on an LCD (Liquid Crystal Display) or monitor mounted on the camera. Further, a digital camera allows image data representative of a picture to be input to a computer.
A current trend in a digital camera art is toward a smaller image sensing device, or smaller pixel size, that enhances the resolution of a picture and reduces the cost of the camera. A decrease in pixel size, however, directly translates into a decrease in the sensitivity of the individual image sensing device. While picture quality may be enhanced if both of resolution and sensitivity are increased, resolution and sensitivity are contrary to each other in the above respect.
In light of the above, nearby pixels may be shifted from each other in order to enhance resolution. Japanese patent laid-open publication No. 340455/1996, for example, discloses an image signal processor including unique data generating means. The data generating means generates pixel data in a lattice pattern on the basis of signal charges read out of a plurality of actual pixels, which adjoin a given pixel and are arranged in a non-lattice pattern. Despite that the image signal processor uses photosensitive cells arranged in a non-lattice pattern, it is capable of outputting pixel data in a lattice pattern that can be adequately input to a computer.
Japanese patent laid-open publication No. 72283/1984 teaches an electronic still camera using a solid-state photosensitive cells, or pixels, that are arranged in a checker pattern and allow video signals to be read out by progressive scanning on a two scanning line basis. The still camera includes a video signal processor including first circuit means and second circuit means. The first circuit means combines video signals on an upper and a lower scanning line by interpolation in the horizontal direction to thereby produce a new single line of video signals. The second circuit means delays the video signals on lower one of the two scanning lines by a single scanning period. The second circuit means then combines the delayed video signals with video signals on upper one of the next two scanning lines by interpolation in the horizontal direction, thereby producing a new single line of video signals.
The first and second circuit means output the respective video signals at the same time. The video signal processor can therefore output signals belonging to an odd-numbered field and an even-numbered field, which are particular to 2:1 interlace scanning, at the same time by a single vertical scanning. This implements a high quality still shot even when the horizontal and vertical directions are replaced with each other, i.e., without regard to the position of a frame with respect to vertical and horizontal directions.
Other various schemes have been proposed to reduce the pixel size, which is spatially sampled, and enhance sensitivity at the same time. For example, Masafumi Inuiya presents references of 1998 and a study thereof in “Image Pickup Characteristics in Megapixels DS Camera”, the Society of Photographic Science and Technology of Japan, Digital Camera Group. Inuiya discusses, e.g., the influence of the pixel size on the image pickup characteristics in relation to sensitivity and S/N (Signal-to-Noise) ratio and an image pickup system using a single color filter. A solid-state image pickup apparatus adopts either one of a primary color pickup system using an red (R), green (G) and blue (B) color filter and a complementary color pickup system using a plurality of complementary colors. Inuiya reports that the complementary color pickup system is superior to the primary color pickup system with respect to the efficient use of incident light.
As for the complementary color pickup system, Japanese patent laid-open publication No. 31688/1983 proposes a solid-state color image pickup apparatus directed toward high resolution. In the apparatus taught in this document, photosensitive cells adjoining each other in the vertical direction are shifted from each other by half a pitch in the horizontal direction. Three color filter segments each are provided with a particular spectral characteristic such that the sum of the outputs of three adjoining photosensitive cells substantially corresponds to a luminance signal. The apparatus is capable of reducing moiré and increasing resolution. More specifically, the apparatus uses a color filter, white (W), yellow (Ye) and cyan (Cy) filter segments arranged in a delta shape on complementary color system and produces a luminance signal from three adjoining pixels (two horizontal lines).
The technologies taught in the above-described laid-open publication Nos. 72283/1984 and 31688/1983 each use a mixed two-line reading system or simultaneous two-line independent reading system belonging to a family of color multiplexing systems. This reading system feeds signal charges derived from incident light to signal lines and then reads out two signal lines by mixing them together. The above technologies both assume a movie and a MOS (Metal Oxide Semiconductor) image sensor.
It is known that progressive scanning (or all pixel read-out) conventional with a CCD (Charge Coupled Device) image sensor is not applicable to a MOS image sensor. Therefore, a picture available with a MOS image sensor is lower in resolution, particularly vertical resolution in the case of a movie, than a picture implemented by progressive scanning. Even the image signal processor proposed in the previously mentioned laid-open publication No. 340455/1996 cannot fully meet the demand for high vertical resolution.